Solid golf ball

ABSTRACT

A solid golf ball with a polyurethane cover is provided which is satisfactory in the moldability of the cover and in ball characteristics such as a shot feeling and a flight distance. The solid golf ball of the invention includes a solid center, an intermediate layer, and a polyurethane cover. A material having a flexural modulus of from 150 to 420 MPa is used for the intermediate layer in order for the golf ball to offer satisfactory shot feeling and flight distance. By the use of a vulcanizate of a rubber composition containing a diphenyl disulfide for the solid center, the golf ball can exhibit an increased flight distance. In order to enhance the moldability of the cover a urethane prepolymer having a residual isocyanate monomer content of not more than 0.5% by mass is used.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a multi-piece solid golf ball comprising a solid center, an intermediate layer and a polyurethane cover.

[0003] 2. Description of the Related Art

[0004] An ionomer resin cover is mainly used for a cover of a solid golf ball employing vulcanized rubber body as a solid core because the ionomer resin cover has superior durability. The golf ball with the ionomer resin cover, however, gives a golfer a larger impact upon shot than do a golf ball with Balata rubber cover, and hence is likely to impart an inferior shot feeling to the golfer.

[0005] In attempt to improve the shot feeling of the golf ball with the ionomer cover, Japanese Patent No. 2709950, for example, has proposed a mixed ionomer cover formed from a mixture of a hard ionomer comprising a sodium salt or zinc salt of an olefin-unsaturated carboxylic acid copolymer and a soft ionomer comprising a sodium salt or zinc salt of an olefin-unsaturated carboxylic acid-unsaturated carboxylic acid ester terpolymer. Blending of such a soft ionomer in a cover enables to render the shot feeling soft, but on the other hand, sacrifices the advantages inherent to the ionomer cover; for example, the abrasion resistance (scratching resistance) as well as the repulsion property of the golf ball.

[0006] In recent years, polyurethane has been focused on as an inexpensive cover material that imparts to the golfer a shot feeling analogous to that imparted by the Balata cover and has higher durability than the Balata cover. For example, the golf ball with the following polyurethane cover has been proposed.

[0007] In view of a problem that the molding of the polyurethane cover is difficult because the reaction between a urethane prepolymer and a polyamine curing agent proceeds rapidly to cause a steep increase in viscosity, Japanese Patent No. 2662909 has disclosed a polyurethane cover formed by curing a urethane prepolymer with a slow-reactive polyamine curing agent. In Japanese Patent No. 2662909, the steep increase in viscosity due to the rapid reaction between the urethane prepolymer and the polyamine curing agent is prevented by the use of the slow-reactive polyamine curing agent. However, in case of some kinds of urethane prepolymer, some kinds of curing agent, some combinations of urethane prepolymer and curing agent, or the like, the increase in viscosity is still rapid, and thus the molding of the cover is difficult. Even if the molding of the cover is possible, a golf ball with the resulting cover is not sufficient in terms of the repulsion property, spin performance and abraision reisistance(scrathcing resistance). Thus, further improvements are desired.

[0008] Japanese unexamined patent publication No. H09-215778 has proposed a cover formed using a thermoplastic polyurethane elastomer. Such a thermoplastic polyurethane elastomer used as a cover material is superior in moldability to the two-part curing type polyurethane covers, but are inferior in wear resistance, tear strength and scratching resistance to the two-part curing type polyurethane cover or the ionomer cover, because of the lack of three-dimensionally crosslinking points.

[0009] Japanese unexamined patent publication No. 2000-513609 has proposed a golf ball with a double-layered cover. The double-layered cover comprises a thermosetting polyurethane cover having a thickness of less than 0.127 cm as an outer layer and a cover material having a high flexural modulus as an inner layer. This golf ball provides a satisfactory flight distance upon a driver shot for long distance and exhibits satisfactory spin performance upon a wedge shot for short distance. However, the golf ball has a problem of a hard feeling imparted to the golfer upon shot (hereinafter referred to as “hard shot feeling”) because of the too hard inner cover material, and the like problems.

[0010] The present invention has been made in view of the foregoing circumstances. Accordingly, it is an object of the present invention to provide a solid golf ball with a polyurethane cover, which satisfies the moldability of the cover, and the, shot feeling and the flight distance.

SUMMARY OF THE INVENTION

[0011] According to the present invention, there is provided a solid golf ball comprising a solid center, an intermediate layer, and a polyurethane cover, wherein: the solid center has an outer diameter of from 36.7 to 40.8 mm; the intermediate layer has a thickness of from 0.5 to 1.5 mm and is formed from a material having a flexural modulus of from 150 to 420 MPa; and the polyurethane cover has a thickness of from 0.5 to 1.5 mm and is formed from a cured product of a composition comprising an isocyanate group-terminated urethane prepolymer and an aromatic polyamine compound, the cured product having a Shore D hardness of from 35 to 55.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The present invention will now be described in detail. The solid golf ball of the present invention is characterized by comprising a solid center, an intermediate layer, and a polyurethane cover, wherein: the solid center has an outer diameter of from 36.7 mm to 40.8 mm; the intermediate layer has a thickness of from 0.5 to 1.5 mm and is formed from a material having a flexural modulus of from 150 MPa to 420 MPa; and the polyurethane cover has a thickness of from 0.5 mm to 1.5 mm and is formed from a cured product of a composition comprising an isocyanate group-terminated urethane prepolymer and an aromatic polyamine compound (hereinafter referred to as “polyurethane cover composition”), the cured product having a Shore D hardness of from 35 to 55.

[0013] First of all, the polyurethane cover used in the present invention is described. The polyurethane cover is formed from the cured product of the polyurethane cover composition, which comprises the isocyanate group-terminated urethane prepolymer and the aromatic polyamine compound. The isocyanate group-terminated urethane prepolymer used in the present invention may include any isocyanate group-terminated urethane prepolymer, without any particular limitation, as long as it has at least two isocyanate groups in the urethane prepolymer molecular chain thereof. The isocyanate group-termnated urethane prepolymer can have each isocyanate group at the terminal of the backbone chain of the urethane prepolymer molecular chain or at the terminal of a side chain thereof, without limitation.

[0014] The isocyanate group-terminated urethane prepolymer is prepared by allowing a polyol and a polyisocyanate compound to react with each other under the condition that the isocyanate group of the polyisocyanate compound is in excess relative to the hydroxyl group of the polyol in molar ratio. In the present invention, the residual polyisocyanate monomer content contained in the isocyanate group-terminated urethane prepolymer is preferably not more than 0.5% by mass, more preferably not more than 0.1% by mass. The “residual polyisocyanate monomer”, as used herein, means an unreacted polyisocyanate compound remaining in the isocyanate group-terminated urethane prepolymer. For example, an excess of the polyisocyanate compound used for the preparation of the isocyanate group-terminated urethane prepolymer partially remains unreacted therein. If the residual polyisocyanate monomer content is more than 0.5% by mass, a precipitate is likely to be generated in the polyurethane cover composition. Although the mechanism of the formation of the precipitate is not apparent, it is presumed that the reaction product of the residual polyisocyanate monomer and the polyamine curing agent precipitates. The precipitation causes the non-uniform reaction between the isocyanate group-terminated urethane prepolymer and the aromatic polyamine compound, resulting in difficulty in manufacturing a uniform polyurethane cover. Non-uniformity of the polyurethane cover affects the durability of the cover; particularly, it causes to lower the abrasion resistance (scratching resistance) of the cover. In an extreme case, the polyurethane cover composition is cured before manufacturing the polyurethane cover, and thus it is substantially impossible to mold the polyurethane cover composition.

[0015] The residual polyisocyanate monomer content of the isocyanate group-terminated urethane prepolymer is defined by the expression: (the mass of the residual polyisocyanate monomer in the isocyanate group-terminated urethane prepolymer/the total mass of the isocyanate group-terminated urethane prepolymer)×100 and can be determined by gas chromatography. Examples of specific isocyanate group-terminated urethane prepolymers having a residual polyisocyanate monomer content of not more than 0.5% by mass are ADIPRENE LF600D, ADIPRENE LF800A, ADIPRENE LF900A, and ADIPRENE LF950A, which are commercially available from UNIROYAL CO.

[0016] The isocyanate group-terminated urethane prepolymer comprises a conventional polyisocyanate compound as an isocyanate component. Examples of the polyisocyanate compound are an aromatic diisocyanate such as 2,4-tolylene diisocyanate, 2,6-tolylene diisocyanate, or mixtures thereof (TDI), 4,4′-diphenylmethane diisocyanate or a polynuclear compound thereof, 1,5-naphthylene diisocyanate (NDI), 3,3′-bitolylene-4,4′-diisocyanate (TODI), xylylene diisocyanate (XDI), and paraphenylene diisocyanate (PPDI); and an alicyclic or aliphatic diisocyanate such as 4,4′-dicyclohexylmethane diisocyanate (hydrogenated MDI), hexamethylene diisocyanate (HDI) and isophorone diisocyanate (IPDI). These may be used either alone or as a mixture of at least two of them. Preferably, the isocyanate component comprises at least one selected from the group consisting of tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate (MDI), and paraphenylene diisocyanate (PPDI) because the resulting polyurethane cover has favorable mechanical properties and because the golf ball with the resulting polyurethane cover has the sufficient repulsion property, weather resistance and water resistance.

[0017] The isocyanate group-terminated urethane prepolymer contains any conventional polyol as a polyol component. The polyol has no limitation as to whether the polyol has low-molecular-weight or high-molecular-weight, as long as it has a plurality of hydroxyl groups. Examples of the low-molecular-weight polyols are a diol such as ethylene glycol, diethylene glycol, triethylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, and 1,6-hexanediol; and a triol such as glycerin, trimethylolpropane, and hexanetriol. Examples of the high-molecular-weight polyols are a polyetherpolyol generally resulting from the reaction between an initiator having active hydrogen and alkylene oxide; a condensed polyesterpolyol generally resulting from the condensation between a dibasic acid, such as adipic acid, and a glycol or a triol; a lactone polyesterpolyol generally resulting from ring opening polymerization of a lactam such as ε-caprolactam; a polycarbonate diol generally synthesized using a cyclic diol; and a polymer polyol, such as an acrylic polyol, generally prepared by introducing an appropriate hydroxyl group into an acrylic copolymer. Examples of specific polyetherpolyols are polyoxyethylene glycol(PEG), polyoxypropylene glycol (PPG), and polyoxytetramethylene glycol (PTMG). Examples of specific condensed polyesterpolyols are polyethylene adipate (PEA), polybutylene adipate (PBA), and polyhexamethylene adipate (PHMA). Examples of specific lactone polyesterpolyols are poly-ε-caprolactone (PCL). In view of their superior repulsion property and water resistance, polyetherpolyols are preferable. Use of polyoxytetramethylene glycol is particularly preferable.

[0018] The isocyanate group-terminated urethane prepolymer is preferably at least one selected from the group consisting of a tolylene diisocyanate urethane prepolymer, a MDI urethane prepolymer, and a PPDI urethane prepolymer. The “tolylene diisocyanate urethane prepolymer”, as used herein, means the isocyanate group-terminated urethane prepolymer resulting from the reaction between tolylene diisocyanate or a polyisocyanate comprising tolylene diisocyanate as a major component and a polyol (preferably polyoxytetramethylene glycol). The “MDI urethane prepolymer”, as used herein, means the isocyanate group-terminated urethane prepolymer resulting from the reaction between MDI or a polyisocyanate comprising MDI as a major component and a polyol (preferably polytetramethylene glycol). The “PPDI urethane prepolymer”, as used herein, means the isocyanate group-terminated urethane prepolymer resulting from the reaction between PPDI or a polyisocyanate comprising PPDI as a major component and a polyol (preferably polytetramethylene glycol).

[0019] The aromatic polyamine compound used in the present invention has no limitation, as long as it has at least two amino groups directly or indirectly bonded to an aromatic ring. Herein, the “indirectly bond to the aromatic ring”, for example, means that the amino group is bonded to the aromatic ring through a sulfide bond or a lower alkylene bond. Further, the aromatic polyamine compound may include a monocyclic aromatic polyamine compound having at least two amino groups bonded to one aromatic ring or a polycyclic aromatic polyamine compound having at least two aminophenyl groups each having at least one amino group bonded to one aromatic ring.

[0020] Examples of the monocyclic aromatic polyamine compounds are a type such as phenylenediamine, toluenediamine or diethyltoluenediamine where amino groups are directly bonded to an aromatic ring; a type such as dimethylthiotoluenediamine where amino groups are bonded to an aromatic ring through a sulfide bond; and a type such as xylylenediamine where amino groups are bonded to an aromatic ring through a lower alkylene group. Use of dimethylthiotoluenediamine or diethyltoluenediamine is particularly preferable because they have moderate reactivity with the isocyanate group-terminated urethane prepolymer.

[0021] The polycyclic aromatic polyamine compound may include polyaminobenzene having at least two aminophenyl groups directly bonded to each other or a compound having two aminophenyl groups bonded to each other through a lower alkylene group or an alkylene oxide group. Among them, diaminodiphenylalkane having two aminophenyl groups bonded to each other through a lower alkylene group is preferable. Typically preferred are 4,4′-diaminodiphenylmethane and derivatives thereof. With such a compound, benzene nuclei forming hard segments can be arranged side-by-side linearly on a plane if the compound is of para-form and has a not very long molecular chain intervening between aminophenyl groups, and hence it is possible to efficiently make use of the intermolecular cohesive energy based on urethane bond, urea bond, hydrogen bond between benzene nuclei, thus resulting in the improved repulsion property, tensile strength and tear strength. For this reason, the resulting cover tends to be improved in cover strength and cover durability such as abraision resistance(scrathcing resistance).

[0022] Examples of derivatives of 4,4′-diaminodiphenylmethane are 3,3′-dichloro-4,4′-diaminodiphenylmethane, 3,3′-dimethyl-5,5′-diethyl-4,4′-diaminodiphenylmethane, 3,3′,5,5′-tetramethyl-4,4′-diaminodiphenylmethane, 3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane, 3,3′,5,5′-tetraisopropyl-4,4′-diaminodiphenylmethane, 3,3′-dimethyl-5,5′-diisopropyl-4,4′-diaminodiphenylmethane, 3,3′-diethyl-5,5′-diisopropyl-4,4′-diaminodiphenylmethane, 3,3′-dimethyl-5,5′-di-t-butyl-4,4′-diaminodiphenylmethane, 3,3′-dichloro-5,5′-diethyl-4,4′-diaminodiphenylmethane, 2,2′-dichloro-3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane, and 2,2′,3,3′-tetrachloro-4,4′-diaminodiphenylmethane. Among them, 2,2′-dichloro-3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane is particularly preferable because its toxicity is relative low.

[0023] Though the blending ratio between the amount of the isocyanate group-terminated urethane prepolymer and the amount of the aromatic polyamine compound contained in the urethane cover composition is not particularly limited, they are preferably mixed so that the molar ratio of the amino group to the isocyanate group (NH₂/NCO) falls within the range of from 0.7 to 1.2, preferably from 0.8 to 1.05, more preferably from 0.85 to 1.0. If the ratio is adjusted so that a cured product of the resulting polyurethane composition has buret crosslinking or allophanate crosslinking, a polyurethane cover to be formed from the cured product has a three-dimensionally crosslinked structure, and hence is excellent in durability and abrasion resistance.

[0024] The urethane cover composition used in the present invention may further contain any conventionally catalyst known in a urethane reaction. Examples of the catalysts are a monoamine such as triethylamine and N,N-dimethylcyclohexylamine; a polyamine such as N,N,N′,N′-tetramethylethylenediamine and N,N,N′,N″,N″-pentamethyldiethylenetriamine; a cyclic diamine such as 1,8-diazabicyclo[5,4,0]-7-undecene (DBU) and triethylenediamine; and a tin catalyst such as dibutyltin dilaurylate and dibutyltin diacetate. Among them, 1,8-diazabicyclo[5,4,0]-7-undecene (DBU) and triethylenediamine are preferable. In the case that the reactivity of the isocyanate group is too high, a retarding catalyst may be used. Examples of the retarding catalyst are organic carboxylic acids such as acetic acid, azelaic acid, and adipic acid. Among them, azelaic acid is preferably used.

[0025] As required, the polyurethane cover composition may further contain a filler such as barium sulfate, a coloring agent such as titanium dioxide, and other additives such as a dispersant, antioxidant, ultraviolet absorber, light stabilizer, fluorescent material, and fluorescent brightener, as long as any desired property is not deteriorated.

[0026] The preparation of the polyurethane cover composition preferably needs to mix the isocyanate group-terminated urethane prepolymer and the aromatic polyamine compound homogeneously. Thus, the two components are preferably mixed at such a temperature that the aromatic polyamine compound is in a molten state. For example, the urethane prepolymer is heated to a temperature close to such a temperature that the aromatic polyamine compound turned into a molten state and then mixed with the aromatic polyamine compound in a molten state.

[0027] With respect to the polyurethane cover of the golf ball of the present invention, the hardness of the cured product of the foregoing polyurethane cover composition (the hardness of the cured product itself may be referred to as “slab hardness”) is preferably not less than 35, more preferably not less than 40 by Shore D hardness. If the hardness is less than 35 by Shore D, the resulting cover is so soft that the golf ball with this cover is easy to spin, exhibits a lowered repulsion property and also exhibits a lower abrasion resistance (scratching resistance) against the shot by an iron or a sand wedge. Since the cover becomes harder as the Shore D hardness (slab hardness) of the cured product increases, the cover having too high a hardness results in a golf ball which imparts a hard shot feeling to the golfer and which cannot ensure satisfactory controllability in approach shots because of too low spin rate. Therefore, the Shore D hardness of the cured product forming the polyurethane cover is preferably not more than 55, more preferably not more than 52. The polyurethane cover preferably has the thickness of from 0.7 mm to 1.3 mm.

[0028] The intermediate layer of the solid golf ball of the present invention is formed from a material having a flexural modulus of from 150 MPa, preferably from 200 MPa, to 420 MPa, preferably to 400 MPa. The flexural modulus of the intermediate layer is indicating the elasticity of the material forming the intermediate layer and is a factor particularly influencing the flight distance of the golf ball. If the intermediate layer has the flexural modulus of lower than 150 MPa, the resulting golf ball exhibits a lowered repulsion property and hence offers a shorter flight distance. On the other hand, if the intermediate layer has the flexural modulus of higher than 420 MPa, the resulting golf ball is so hard that it imparts an inferior shot feeling to the golfer. In the present invention, the flexural modulus is therefore adjusted to fall within the aforementioned range, in view of that the flexural modulus of the intermediate layer reflects the behavior of the deformation of the golf ball that will occur upon shot. If the flexural modulus of the intermediate layer is so adjusted, the intermediate layer, for example, has a Shore D hardness of not less than 55, preferably not less than 60, and has a Shore D hardness not more than 70, preferably not more than 68. If the Shore D hardness of the intermediate layer is less than 55, the intermediate layer is so soft that the resulting golf ball exhibits a lowered repulsion property, while if it is more than 70, the intermediate layer is so hard that the resulting golf ball tends to impart a poor shot feeling to the golfer.

[0029] The material forming the intermediate layer is not particularly limited so long as the flexural modulus thereof falls within the aforementioned range. Examples of the materials are a thermoplastic resin such as a polyurethane resin, an ionomer resin, nylon, and polyethylene; and a thermoplastic elastomer such as a polystyrene elastomer, a polyolefin elastomer, a polyurethane elastomer, and a polyester elastomer. Among them, the ionomer resin is particularly preferable in view of its superior repulsion property and durability. Examples of the ionomer resin are one prepared by neutralizing at least a part of carboxyl groups in a copolymer of ethylene and α,β-unsaturated carboxylic acid with a metal ion, and one prepared by neutralizing at least a part of carboxyl groups in a terpolymer of ethylene, α,β-unsaturated carboxylic acid and α,β-unsaturated carboxylic acid ester with a metal ion. Examples of the α,β-unsaturated carboxylic acids are acrylic acid, methacrylic acid, fumaric acid, maleic acid, and crotonic acid. Among them, acrylic acid and methacrylic acid are particularly preferable. Examples of the α,β-unsaturated carboxylic acid esters are methyl ester, ethyl ester, propyl ester, n-butyl ester, isobutyl ester and the like of acrylic acid, methacrylic acid, fumaric acid, maleic acid and the like. Among them, acrylic ester and methacrylic ester are particularly preferable. The metal ion for neutralizing the carboxylic group includes sodium ion, potassium ion, lithium ion, magnesium ion, calcium ion, zinc ion, barium ion, aluminum ion, tin ion, zirconium ion, and cadmium ion. Among them, sodium ion, zinc ion and magnesium ion are preferable in view of improving the repulsion property and durability. In the present invention, the acid value and the metal ion content of the ionomer resin are not particularly limited. Further, it is possible to use a single or a mixture of plural ionomer resins so that the flexural modulus of the resulting intermediate layer falls within the aforementioned range.

[0030] The intermediate layer may further contain a specific gravity adjusting agent such as barium sulfate or tungsten. In the case that the intermediate layer contains a filler, it is preferable to adjust the flexural modulus of the intermediate layer containing the filler so as to fall within the aforementioned range. The intermediate layer according to the present invention has a thickness of from 0.5 mm to 1.5 mm, preferably from 0.7 mm to 1.2 mm. If the thickness of the intermediate layer is less than 0.5 mm, the resulting golf ball has a lowered repulsion property, while if it is more than 1.5 mm, the resulting golf ball imparts a hard shot feeling to the golfer and has a lowered repulsion property.

[0031] The solid center of the solid golf ball according to the present invention has an outer diameter of from 36.7 mm to 40.8 mm, preferably from 38.0 mm to 40.0 mm. If the outer diameter of the center is less than 36.7 mm, either of the intermediate layer and the polyurethane cover becomes so thick as to cause the lowered repulsion property and a poor shot feeling. On the other hand, if the outer diameter of the center is more than 40.8 mm, the intermediate layer and the polyurethane cover become so thin that the resulting golf ball cannot enjoy the effects which would otherwise be provided by the intermediate layer and the polyurethane cover. There is no particular limitation on the composition of the solid center, as long as its outer diameter is within the range described above. Usually, the solid center is a vulcanized body of a rubber composition, which is generally used for the centers of solid golf balls. The rubber composition for the solid center (hereinafter referred to as “center rubber composition” as the case may be) preferably comprises a base rubber, an organic peroxide as a crosslinking agent, an organic sulfur compound, and a co-crosslinking agent. As required, the center rubber composition may further contain additives such as a specific gravity adjusting agent, antioxidant, and color powder. Examples of the base rubbers are butadiene rubber (BR), ethylene-propylene-diene terpolymer (EPDM), isoprene rubber (IR), styrene-butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR). Among them, butadiene rubber, particularly cis-1,4-polybutadiene, is preferable in view of its superior repulsion property. A preferred embodiment of the present invention employs a high-cis polybutadiene rubber having cis-1,4 bonds in a proportion of not less than 40%, preferably not less than 70%, more preferably not less than 90%.

[0032] Examples of the organic peroxides for use in the present invention are dicumyl peroxide, 1,1-bis(t-butylperoxy)-3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butylperoxy)hexane, and di-t-butyl peroxide. Among them, dicumyl peroxide is preferable. The amount of the organic peroxide to be blended in the center rubber composition is preferably from 0.1 to 3.0 parts by mass, more preferably from 0.2 to 2.0 parts by mass based on 100 parts by mass of the base rubber.

[0033] The co-crosslinking agent used in the present invention includes, for example, an α,β-unsaturated carboxylic acid or a metal salt thereof. Typically preferred is α,β-unsaturated carboxylic acid having 3 to 8 carbon atoms such as acrylic acid and methacrylic acid. As a metal forming metal salts of the α,β-unsaturated carboxylic acid, monovalent or divalent metals such as zinc and magnesium is preferably used. Among them, zinc acrylate is preferable because it can impart the higher repulsion property to the golf ball. The amount of the co-crosslinking agent to be blended in the center rubber composition is preferably from 15 to 50 parts by mass, more preferably 20 to 40 parts by mass based on 100 parts by mass of the base rubber.

[0034] The center rubber composition used in the present invention preferably further contains an organic sulfur compound (diphenyl disulfide derivative) represented by the following formula:

[0035] wherein each of X¹ to X¹⁰ is any one selected from the group consisting of a hydrogen atom, a chlorine atom, a bromine atom, an iodine atom, and a cyano group, and X¹ to X¹⁰ may be same or different each other. Although the diphenyl disulfide derivative can have a symmetric or asymmetric structure, it is preferable that the diphenyl disulfide derivative has the symmetric structure (bis-structure). Examples of the diphenyl disulfide derivatives are diphenyl disulfide; mono-substituted diphenyl disulfide such as bis(4-chlorophenyl) disulfide, bis(3-chlorophenyl) disulfide, bis(4-bromophenyl) disulfide, bis(3-bromophenyl) disulfide, bis(4-fluorophenyl) disulfide, bis(4-iodophenyl) disulfide, and bis(4-cyanophenyl) disulfide; di-substituted diphenyl disulfide such as bis(2,5-dichlorophenyl) disulfide, bis(3,5-dichlorophenyl) disulfide, bis(2,6-dichlorophenyl) disulfide, bis(2,5-dibromophenyl) disulfide, bis(3,5-dibromophenyl) disulfide, bis(2-chloro-5-bromophenyl) disulfide, and bis(2-cyano-5-bromophenyl) disulfide; tri-substituted diphenyl disulfides such as bis(2,4,6-trichlorophenyl) disulfide and bis(2-cyano-4-chloro-6-bromophenyl) disulfide; tetra-substituted diphenyl disulfide such as bis(2,3,5,6-tetrachlorophenyl) disulfide; and penta-substituted diphenyl disulfide such as bis(2,3,4,5,6-pentachlorophenyl) disulfide and bis(2,3,4,5,6-pentabromophenyl) disulfide. These diphenyl disulfides can exert some influence upon the vulcanized state of the vulcanized rubber to enhance the repulsion property of the resulting golf ball. Among them, diphenyl disulfide and bis(pentachlorophenyl) disulfide are particularly preferable from the viewpoint that they can impart the higher repulsion property to the golf ball.

[0036] The conditions for vulcanizing the center rubber composition can be established appropriately depending upon the formulation of the rubber composition. It is desirable that the center rubber composition is vulcanized for 10 to 60 minutes at the temperature of not lower than 130° C., more preferably not lower than 160° C., and not higher than 200° C., more preferably not higher than 180° C. According to another preferred embodiment of the present invention, the center rubber composition is vulcanized by heating at a relatively low temperature ranging between 130° C. and 150° C. for 20 to 40 minutes and then at a higher temperature ranging between 160° C. and 180° C. for 5 to 15 minutes. The two-step vulcanization provides a center having uniform hardness between the surface portion and the internal portion of the solid center.

[0037] The solid center preferably has the deformation amount of not less than 2.8 mm, more preferably not less than 2.85 mm, and not more than 3.5 mm, more preferably not more than 3.3 mm, when applying a load from 98N (10 kgf) as an initial load to 1275N (130 kgf) as a final load. If the deformation amount is less than 2.8 mm, the center becomes so hard that the center greatly influences the shot feeling, thus resulting in a larger impact given to the golfer upon shot (that is, a harder shot feeling imparted to the golfer). On the other hand, if the deformation amount is more than 3.5 mm, the center becomes so soft that the resulting golf ball cannot provide a satisfactory flight distance.

[0038] The solid golf ball of the present invention preferably has the deformation amount of not less than 2.7 mm, more preferably not less than 2.8 mm, and not more than 3.5 mm, more preferably not more than 3.3 mm, when applying a load from 98N (10 kgf) as an initial load to 1275N (130 kgf) as a final load. If the deformation amount is less than 2.7 mm, the golf ball imparts a harder shot feeling to the golfer, while if the deformation amount is more than 3.5 mm, the golf ball has a lowered repulsion property and hence cannot exhibit a satisfactory flight distance.

[0039] The solid golf ball of the present invention can be manufactured by a well-known process which is conventionally employed for manufacturing the golf ball having a polyurethane cover. For example, the solid center of vulcanizing-molded body is covered with the intermediate layer to form a solid core. In covering the solid center with the intermediate layer, it is possible to employ a process including: forming the intermediate layer material into two hemispherical half shells; enveloping the solid center with the two half shells; and then pressure-molding. Alternatively, it is possible to employ a process including injection-molding the intermediate layer material directly onto the solid center to cover the solid center. Subsequently, the obtained solid core is hold in the hemispherical mold, and then the polyurethane cover composition is charged into the hemispherical mold. The hemispherical mold holding the solid core is inverted to mate with another hemispherical mold into which the polyurethane cover composition has been charged. Finally, the polyurethane cover composition is cured to mold the cover. The curing reaction of the polyurethane cover composition is preferably conducted at the temperature of from 30° C. to 120° C. for 2 to 60 minutes, preferably at the temperature of from 50° C. to 80° C. for 5 to 30 minutes. As required, the polyurethane cover is formed with a multiplicity of dimples at the surface thereof during molding the cover. Further, the golf ball of the present invention is usually provided with paint finish, a marking stamp and the like to enhance the appearance and the commercial value thereof before it is launched into the market.

[0040] In the present invention, the polyurethane cover may be composed of a single layer or plural layers.

EXAMPLES

[0041] The following examples illustrate the present invention, however these examples are intended to illustrate the invention and are not to be construed to limit the scope of the invention. Many variations and modifications of such examples will exist without departing from the scope of the inventions. Such variations and modifications are intended to be within the scope of the invention.

[0042] [Measurement and Evaluation Methods]

[0043] 1. Compressive Deformation Amount (mm)

[0044] The deformation amount (the deformation amount of shrinkage along the loading direction) was measured when applying a load from 98N as an initial load to 98 N as a final load of 1275 N in terms of each of the solid center and the solid golf ball.

[0045] 2. Repulsion Coefficient

[0046] An aluminum cylindrical body having a weight of 198.4 g was allowed to collide with each solid golf ball at the speed of 45 m/sec. The respective speeds of the cylindrical body and the solid golf ball before and after the collision were measured, and the repulsion coefficient of each solid golf ball was calculated from the respective speeds thus measured and the respective masses of the cylindrical body and the solid golf ball. For each solid golf ball, 12 samples were measured. The average of the measured values was regarded as the repulsion coefficient of each solid golf ball. The repulsion coefficient of each solid golf ball was represented as an index relative to 100 representing the repulsion coefficient of the golf ball No. 12.

[0047] 3. Flight Distance (Carry) and Spin Rate(rpm)

[0048] Each golf ball was hit with a titan-head wood driver (W#1) attached to a swing robot manufactured by TRUETEMPER CO. at the head speed of 45 m/sec. The flight distance from the hitting point to the point at which the ball fell to the ground, was measured. The spin rate was determined by hitting each golf ball with a sand wedge attached to the swing robot at the head speed of 20 m/sec.

[0049] 4. Shot Feeling

[0050] An actual hitting test was carried out by ten golfers including professional golfers and high-level amateur golfers with a W#1 club. The shot feeling of each golf ball was evaluated based on the magnitude of the impact upon shot according to the following rating criteria. The rating category to which the largest number of golfers agreed for each golf ball was determined as the shot feeling of the golf ball.

[0051] Rating Criteria:

[0052] “Excellent”: A golf ball gave a very small impact upon shot and imparted a very soft shot feeling;

[0053] “Good”: A golf ball gave a small impact upon shot and imparted a soft shot feeling;

[0054] “Fair”: A golf ball gave an ordinary impact upon shot; and

[0055] “Poor”: A golf ball gave a large impact upon shot, hence imparted a poor shot feeling.

[0056] 5. Flexural Modulus

[0057] According to JIS-K7171, a 4 mm-thick, 20 mm-long and 10 mm-wide plate formed from each intermediate layer composition was subjected to a three-point flexural test. The flexural modulus of each plate was determined from stresses corresponding to strain ε₁=0.0005 and strain ε₂=0.0025.

[0058] 6. Hardness of the Cured Product of Polyurethane Cover Composition (Slab Hardness)

[0059] Each polyurethane cover composition was formed into sheets each having a thickness of about 2 mm by hot press molding and the resulting sheets were conserved at 23° C. for two weeks. At least three of the sheets were stacked one upon another to remove the influence of the measuring substrate on which the sheets were placed, and the stack was subjected to the measurement using a spring-type Shore D hardness tester prescribed by ASTM-D2240.

[0060] 7. Abrasion Resistance (Scratching Resistance)

[0061] Two portions of each golf ball were each hit once using a commercially available pitching wedge attached to the swing robot at the head speed of 36 m/sec. The condition of each of the two portions thus hit was visually observed and rated on five levels according to the following criteria. The worse result of the two portions was regarded as the result of the golf ball.

[0062] Rating Criteria:

[0063] “Good”: The surface of the golf ball had few flaws, which were at an unnoticeable degree;

[0064] “Fair”: The surface of the golf ball had clearly noticeable flaws and slight nap; and

[0065] “Poor”: The surface of the golf ball was considerably shaved and conspicuously napped.

[0066] [Manufacture of Golf Ball]

[0067] Solid center rubber compositions of the formulation shown in Table 1 were each subjected to vulcanization at 160° C. for 30 minutes to form ten types of solid centers S1 to S10. TABLE 1 Type of Solid Center Composition S1 S2 S3 S4 S5 S6 S7 S8 S9 S10 BR18 100 100 100 100 100 100 100 100 100 100 Zinc acrylate 33 33 33 33 33 33 33 33 33 33 Zinc oxide 14.8 14.2 16.6 16.6 16.6 16.6 16.6 16.7 13.4 19.6 Diphenyl disulfide 0.5 0.5 0.5 0.3 1 — — — 0.5 0.5 Bis(pentachlorophenyl) — — — — — 0.5 — — — — disulfide Dibenzyl disulfide — — — — — — 0.5 — — — Dicumyl peroxide 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5 0.5

[0068] BR18: high cis-1,4-polybutadiene produced by JSR CO.

[0069] Each of the solid centers thus formed was subjected to the injection-mold using each of the intermediate layer materials shown in Table 2 in order to form a solid core comprising the solid center coated with the intermediate layer. TABLE 2 Material of Flexural Intermediate Layer modulus Type Commercial name (MPa) Note M 1 HIMILAN 1605 370 Mitsui-Dupont Polychemical CO., M 2 HIMILAN 1555 250 Mitsui-Dupont Polychemical CO., M 3 HIMILAN 1856 70 Mitsui-Dupont Polychemical CO., M 4 SURLYN 8140 490 Dupont CO.

[0070] Each of the solid cores thus formed was held in a hemispherical mold having a dimple pattern, and each of four types of polyurethane cover materials C1 to C4 shown in Table 3 was injected into the mold. Subsequently, the mold was inverted and mated with another hemispherical mold having a dimple pattern into which the same type of polyurethane cover material had been injected, and curing reaction of the cover material was conducted to form a polyurethane cover. TABLE 3 Cover composition C1 C2 C3 C4 C5 C6 Isocyanate group terminated urethane prepolymer Adiprene LF800A — — 100 — — — Adiprene LF900A — 100 — — — — Adiprene LF950A 100 — — — — — Adiprene LF600D — — — 100 — — Aromatic polyamine compound Lonzacure M-CDEA 26.1 16.2 12.4 31.2 — — Thermoplastic urethane resin Pandex T1198 — — — — 100  — Ionomer resin SURLYN 8120 — — — — — 50 HIMILAN 1855 — — — — — 50 Filler (Titanium dioxide) 2 2 2 2  2  2 Slab Hardness (Shore D) 54 40 33 62 53 50

[0071] Note on table 3:

[0072] ADIPRENE LF800A: a TDI/PTMG type prepolymer produced by UNIROYAL CO., LTD., NCO content=2.9%, residual tolylene diisocyanate content=not more than 0.1%.

[0073] ADIPRENE LF900A: a TDI/PTMG type prepolymer produced by UNIROYAL CO., LTD., NCO content=3.8%, residual tolylene diisocyanate content=not more than 0.1%.

[0074] ADIPRENE LF950A: a TDI/PTMG type prepolymer produced by UNIROYAL CO., LTD., NCO content=6.1%, residual tolylene diisocyanate content=not more than 0.1%.

[0075] ADIPRENE LF600D: a TDI/PTMG type prepolymer produced by UNIROYAL CO., LTD., NCO content=7.3%, residual tolylene diisocyanate content=not more than 0.1%;

[0076] LONZACURE M-CDEA: 2,2′-dichloro-3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane, produced by LONZA CO.

[0077] PANDEX T1198: a thermoplastic polyurethane elastomer;

[0078] SURLYN 8120: a Na-neutralized ethylene/methacrylic acid ionomer produced by DUPONT CO.

[0079] HIMILAN 1855: a Zn-neutralized ethylene/methacrylic acid ionomer produced by MITSUI-DUPONT POLYCHEMICAL.

[0080] In the cases that thermoplastic urethane resin C5 and ionomer resin C6 were used as cover materials, each cover was formed by injection-molding. Each of resulting golf balls was removed from the mold, deburred, and coated with a white paint and then with a clear paint to give a solid golf ball having a diameter of 42.8 mm and a mass of 45.2 to 45.7 g. These solid golf balls were evaluated as to their flying performance, shot feeling and the like. The results of the evaluations are shown in Tables 4 and 5. TABLE 4 Golf ball No. 1 2 3 4 5 6 7 8 9 10 Solid Center — — — — — — — — — — Type S1 S2 S3 S3 S3 S4 S5 S6 S7 S8 Diameter(mm) 39.6 38.8 38.8 38.8 38.8 38.8 38.8 38.8 38.8 38.8 Deformation (mm) 3.18 3.17 3.16 3.16 3.16 3.12 3.24 3.19 3.09 3.1 Intermediate Layer — — — — — — — — — — Type M1 M1 M2 M1 M1 M1 M1 M1 M1 M1 Flexural Modulus(MPa) 370 370 250 370 370 370 370 370 370 370 Thickness (mm) 0.8 0.8 1.2 1.2 1.2 1.2 1.2 1.2 1.2 1.2 Shore D hardness 67 67 62 67 67 67 67 67 67 67 Cover — — — — — — — — — — Type C1 C1 C1 C1 C2 C1 C1 C1 C1 C1 Thickness (mm) 0.8 1.2 0.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Slab hardness( Shore D) 54 54 54 54 40 54 54 54 54 54 Properties of golf ball — — — — — — — — — — Deformation (mm) 3.06 3.08 3.06 2.98 3.1 2.96 3.09 3.03 2.94 2.95 Repulsion coefficient 108 107 107 109 107 107 108 110 104 105 Flight distance(yard) 235 233 233 236 233 233 235 237 228 230 Spin rate (rpm, with SW) 7000 7000 7100 7000 7200 7000 7000 7000 7000 7000 Shot feeling E E E E E E E E G G Scrathcing resistance G G G G G G G G G G

[0081] Golf balls Nos. 1 to 10 are golf balls satisfying the requirements of the present invention as to the outer diameter of the solid center, the thickness and flexural modulus of the intermediate layer, and the composition, thickness and Shore D hardness of the polyurethane cover. Every golf ball was found to exhibit a high repulsion coefficient and to be excellent in flight distance with a wood (W#1), the spin rate with a sand wedge, and the shot feeling. Golf balls Nos. 1 to 8 each employing a diphenyl disulfide type organic sulfur compound as a vulcanizing agent for the solid center rubber composition were each found to exhibit a higher repulsion property and a longer flight distance than both of golf ball No. 9 employing dibenzyl disulfide and golf ball No. 10 not employing any organic sulfur compound (sulfide compound). Golf ball No. 8 employing bis(pentachlorophenyl) disulfide as an organic sulfur compound was found to exhibit a particularly high repulsion property and a very long flight distance. As can be understood from these results, if diphenyl disulfide type organic sulfur compound is used for the center rubber composition, a golf ball exhibiting a high repulsion property and a long flight distance is obtained. TABLE 5 Golf ball No. 11 12 13 14 15 16 17 18 Solid Center — — — — — — — — Type S9 S3 S3 S3 S3 S3 S3 S10 Diameter (mm) 37.6 38.8 38.8 38.8 38.8 38.8 38.8 37.6 Deformation (mm) 3.15 3.16 3.16 3.16 3.16 3.16 3.16 3.12 Intermediate Layer — — — — — — — — Type M1 M3 M4 M1 M1 M1 M1 M1 Flexural Modulus(MPa) 370 70 490 370 370 370 370 370 Thickness (mm) 0.8 1.2 1.2 1.2 1.2 1.2 1.2 1.8 Shore D hardness 67 58 70 67 67 67 67 67 Cover — — — — — — — — Type C1 C1 C1 C3 C4 C5 C6 C1 Thickness (mm) 1.8 0.8 0.8 0.8 0.8 0.8 0.8 0.8 Slab hardness( Shore D) 54 54 54 33 62 53 50 54 Properties of golf ball — — — — — — — — Deformation (mm) 3.12 3.14 2.85 3.19 2.82 2.99 3.01 2.77 Repulsion coefficient 101 100 111 101 110 104 103 107 Flight distance (yard) 219 222 236 217 236 226 224 232 Spin rate (rpm, with SW) 6900 7300 6800 7400 6200 6700 6400 6800 Shot feeling Fair Poor Poor Poor Poor Fair Fair Poor Scratching resistance Good Fair Poor Fair Poor Poor Poor Good

[0082] Golf ball No. 11 exhibited a lower repulsion property and a shorter flight distance because the thickness of the polyurethane cover was as too large as 1.8 mm. Golf ball No. 12 was too soft and hence exhibited a lower repulsion property and a shorter flight distance, and also imparted an inferior shot feeling because the material used for the intermediate layer had the flexural modulus of 70 MPa. Golf ball No. 13 was the case that a material having a flexural modulus of 490 MPa is used for the intermediate layer. Although this golf ball exhibited a higher repulsion property and a longer flight distance, it imparted a harder shot feeling (inferior shot feeling) to the golfer due to the excessively hard intermediate layer.

[0083] These results indicated that use of the material having the flexural modulus of from 150 to 420 MPa for the intermediate layer was preferable for making the shot feeling and the repulsion coefficient well-balanced and compatible with each other. Golf ball No. 14 of which the polyurethane cover had a slab hardness (Shore D hardness) of 33 imparted an inferior shot feeling to the golfer. Further, golf ball No. 14 exhibited a shorter flight distance because the polyurethane cover was so soft that its repulsion property was lowered. The golf ball No. 15 was the case that the polyurethane cover had a slab hardness (Shore D hardness) of 62. In this case, the cover was so hard that the golf ball imparted a poor shot feeling to the golfer. Further, the spin rate of golf ball No. 15 hit with a sand wedge was reduced because, conceivably, the cover was easy to slip. Golf balls Nos. 16 and 17 employed a thermoplastic urethane resin and an ionomer resin, respectively, as their cover materials. The both golf balls Nos. 16 and 17 did not have the polyurethane cover with the repulsion coefficient as high as the polyurethane cover used in the present invention, and hence exhibited a reduced flight distance. Golf ball No. 18 was the case that the intermediate layer had a thickness of 1.8 mm. In this case, the intermediate layer was so thick that the golf ball imparted a harder shot feeling (or an inferior shot feeling).

[0084] The solid golf ball of the present invention comprises the solid center, the intermediate layer, and the cured polyurethane cover, and is characterized in that: the polyurethane cover has a thickness of from 0.5 to 1.5 mm and a Shore D hardness of from 35 to 55; and the intermediate layer has a thickness of from 0.5 to 1.5 mm and is formed from a material having a flexural modulus of from 150 to 420 MPa. The golf ball thus constructed offers a shot feeling and a flight distance that are balanced well. If the golf ball employs a solid center formed by vulcanizing a center rubber composition containing a diphenyl disulfide, the golf ball can have a further enhanced repulsion property and exhibit a longer flight distance.

[0085] Since the polyurethane cover of the solid golf ball according to the present invention is formed from a polyurethane having a three-dimensionally crosslinked structure such as buret crosslinking or allophanate crosslinking, the polyurethane cover is excellent in durability and abrasion resistance. Particularly, by the use of an isocyanate group-terminated urethane prepolymer having a residual isocyanate monomer content of not more than 0.5% in forming the polyurethane cover, the curing reaction of the polyurethane is allowed to proceed uniformly, and hence it is possible to further enhance the abrasion resistance and durability of the cover. This application is based on Japanese Patent application No.2001-294466 filed on Sep. 26, 2001, the contents of which are hereby incorporated by reference. 

What is claimed is:
 1. A solid golf ball comprising a solid center, an intermediate layer, and a polyurethane cover, wherein the solid center has an outer diameter of from 36.7 mm to 40.8 mm; the intermediate layer has a thickness of from 0.5 mm to 1.5 mm and is formed from a material having a flexural modulus of from 150 to 420 MPa; and the polyurethane cover has a thickness of from 0.5 mm to 1.5 mm and is formed from a cured product of a composition comprising an isocyanate group-terminated urethane prepolymer and an aromatic polyamine compound, the cured product having a Shore D hardness of from 35 to
 55. 2. The solid golf ball according to claim 1, wherein the isocyanate group-terminated urethane prepolymer has a residual isocyanate monomer content of not more than 0.5% by mass.
 3. The solid golf ball according to claim 2, wherein the isocyanate group-terminated urethane prepolymer has a residual isocyanate monomer content of not more than 0.1% by mass.
 4. The solid golf ball according to claim 1, wherein the isocyanate group-terminated urethane prepolymer has an isocyanate component comprising at least one selected from the group consisting of tolylene diisocyanate, 4,4′-diphenylmethane diisocyanate, and paraphenylene diisocyanate.
 5. The solid golf ball according to claim 1, wherein the isocyanate group-terminated urethane prepolymer has an polyol component comprising polyoxytetramethyleneglycol.
 6. The solid golf ball according to claim 1, wherein the aromatic polyamine compound is 2,2′-dichloro-3,3′,5,5′-tetraethyl-4,4′diaminodiphe nylmethane.
 7. The solid golf ball according to claim 1, wherein the solid center has a deformation amount of from 2.8 mm to 3.5 mm when applying a load from 98N as an initial load to 1275 N as a final load, and is formed by vulcanizing a rubber composition which comprises cis-1,4-polybutadiene, an organic peroxide, a co-crosslinking agent, and an organic sulfur compound represented by the general formula:

wherein each of X¹ to X¹⁰ is any one selected from the group consisting of a hydrogen atom, a chlorine atom, a bromine atom, an iodine atom, and a cyano group, and X¹ to X¹⁰ are same or different each other.
 8. The solid golf ball according to claim 7, wherein the organic sulfur compound is diphenyl disulfide or bis(pentachlorophenyl) disulfide.
 9. The solid golf ball according to claim 1, wherein the solid center has the outer diameter of 38 mm to 40 mm.
 10. The solid golf ball according to claim 1, wherein the intermediate layer has a thickness of from 0.7 mm to 1.2 mm and is formed from the material having the flexural modulus of from 200 MPa to 400 MPa.
 11. The solid golf ball according to claim 1, wherein the polyurethane cover has the thickness of 0.7 mm to 1.3 mm.
 12. The solid golf ball according to claim 1, wherein the cured product has the Shore D hardness of from 40 to
 52. 13. The solid golf ball according to claim 1, wherein the material having the flexural modulus of from 150 MPa to 420 MPa is an ionomer resin.
 14. A solid golf ball comprising a solid center, an intermediate layer, and a polyurethane cover, wherein the solid center has an outer diameter of from 36.7 mm to 40.8 mm; the intermediate layer has a thickness of from 0.5 mm to 1.5 mm and is formed from an ionomer resin having a flexural modulus of from 150 MPa to 420 MPa; and the polyurethane cover has a thickness of from 0.5 mm to 1.5 mm and is formed by curing a polyurethane composition, the cured composition having a Shore D hardness of from 35 to 55, wherein the solid center is formed by vulcanizing a rubber composition which comprises cis-1,4-polybutadiene, an organic peroxide, a co-crosslinking agent, and diphenyl disulfide or bis(pentachlorophenyl) disulfide; the polyurethane cover composition comprises an isocyanate group-terminated urethane prepolymer including tolylenediisocyanate as isocyanate component and polyoxytetramethyleneglycol as polyol component, and an aromatic polyamine compound.
 15. The solid golf ball according to claim 14, wherein the isocyanate group-terminated urethane prepolymer has a residual isocyanate monomer content of not more than 0.5% by mass.
 16. The solid golf ball according to claim 15, wherein the aromatic polyamine compound is 2,2′-dichloro-3,3′,5,5′-tetraethyl-4,4′-diaminodiphenylmethane. 